They aren't watching their espresso shot
A serious barista will be paying attention to a lot of variables in order to adjust for the best possible shot. If they're not paying attention to the espresso as it comes out of the machine, they're not focused on quality.
Their milk-steaming makes a ton of noise
You should hear a few crackles, but if it sounds like a steam engine, the barista is probably putting too much air into the milk and it's going to turn out a foamy mess.
They tell you a million facts about the coffee
Newbies will view every customer as an audience at which to spout every coffee fact they know, whereas a pro is more interested in being friendly and giving the customer what they want.
Dan Gentile is a staff writer on Thrillist's National Food and Drink team. He prefers the term poser to phony, but it's mainly because he skateboarded as a teenager. Follow him to a really torn up Element deck at @Dannosphere.